Test Shaft 09

Test Shaft 09, codenamed "Zulu Bunsen," is the last in a group of nine Enrichment Shafts located in the salt mine above which the modern Aperture Science Enrichment Center is built.

Reaching lower than 4375 m up from the deepest parts of the mine, it is the main location of the Portal 2 chapters The Fall and The Reunion.

Test Shaft 09 can be considered as a "timeline" level, as it reveals many details about Aperture Science's background and evolution, from its establishment in the late 1940s to some time before the end of the pre-GLaDOS era, in the late 1980s, through explicit visuals and the prerecorded messages by Cave Johnson and occasionally his assistant Caroline.